In recent years, in an image reading apparatus, such as a personal facsimile, small size and low power consumption light emitting diodes (hereinafter referred to as LEDs) are used as a light source of a reading light source apparatus, since an output of such LEDs and in the sensitivity of a CCD type sensor as a light receiving element have been improved. In a linear light source apparatus, which is equipped with such LEDs as light sources, a linear light source is formed by using a light guiding member, in order to decrease the number of the light sources and to obtain uniform illumination intensity. In particular, light emitted from the light source enters the light guiding member so that the light is led towards a desired direction.
Japanese Patent Application Publication No. 2005-229647 discloses a lighting unit having two light source apparatuses, to each of which a light source is attached at an end portion in a longitudinal direction of a light guiding member, and these two light source apparatuses are arranged to face each other. The light guiding member is made of a resin having high light transmission nature such as acrylic and polycarbonate, or of an optical glass having high light transmission nature, and has a light emitting face at one side portion extending in a longitudinal direction. One or more LEDs are attached as the light source, wherein light that enters the one end face of the light guiding member from the LED is led in a longitudinal direction inside the light guiding member by reflection. At the same time, the light reflected on a light direction changing face, which is formed of a prism face, is emitted from a light emitting face. Each lighting source apparatus is arranged so that a document reading face of a document is irradiated with light that is emitted from each light emitting face, that is, each of them irradiates the same document reading face area. Accordingly, since the document reading face is irradiated with the light from two directions, a shadow is not created even if there are uneven portions formed by folded lines or paper pasting and a good image reading is obtained.
However, in this lighting unit, two light source apparatuses are needed in order to illuminate one irradiation surface. Further, since the light guiding member arranged in a light source apparatus is basically the same shape, the two light source apparatuses are arranged in a state where they are apart from each other at a predetermined distance, so that light directions may become symmetrical. As a result, the entire lighting unit becomes large in size.
In view of the above, described below is a light guiding member and a bifurcated linear light source apparatus, capable of irradiating a document reading face with light from two directions thereof, and capable of realizing the miniaturization of the lighting unit. Japanese Patent Application Publication No. 2008-216409 teaches the technology.
FIG. 8 is an explanatory cross sectional view of the fundamental structure of a light source apparatus according to Japanese Patent Application Publication No. 2008-216409. A light guiding member 81 is formed in a shape of rod whose body is φ5-φ6, and whose full length is 320 mm, and has light direction changing faces 82a and 82b that are formed of two prisms on side faces that extending in a longitudinal direction thereof, and corresponding to a bottom portion. The light guiding member 81 has light emitting faces 83a and 83b which are formed on a common curved surface, right above a traveling direction of the light emitted from the light direction changing faces 82a and 82b. As shown in FIG. 8, the two light direction changing faces 82a and 82b are formed so that the directions, in which the light advances, may intersect, that is, configured so that a line segment 84a, which connects the center of one light direction changing face 82a and a center of light emitting face 83a to each other, intersects with a line segment 84b that connects the center of the other side reflective surface 82b and the center of the other side light emitting face 83b to each other. A light source comprising, for example, light emitting diodes (LED), is located in one end surface of the light guiding member 81 and at an exact point where the line segments 84a and 84b intersect, and is configured so that when light emitted from the light source enters the end surface of the light guiding member 81, the light is led in a length direction, and efficiently enters the light direction changing faces 82a and 82b. 
In the example of the figure, the light reflected on the light direction changing face 82a is transmitted inside the light guiding member 81 so as to be emitted from the light emitting face 83a, and emitted in a direction shown by an arrow P, whereby a document reading face, that is, an irradiation area 86 is irradiated with the light. And the light reflected on the light direction changing face 82b is transmitted inside the light guiding member 81, so as to be emitted from the light emitting face 83b, whereby the light is emitted in a direction shown by an arrow Q.
And a reflective mirror 85 is arranged to face the light guiding member 81, and the light in the direction shown by the arrow Q is directed in a direction shown by an arrow R, so that an irradiation surface is irradiated with the light from two directions (the direction of P, the direction of R) symmetrically with respect to an axis L that passes through the irradiation surface. Therefore, even one light source apparatus can irradiate the light irradiation face 86 from the two directions with the light. As a result, while it is possible to attain highly precise image scanning, it is possible to realize the miniaturization of the lighting unit.